The 3.1 μm absorption feature on asteroids (24) Themis and (65) Cybele is not due to surface water ice

Previous research on Asteroids (24) Themis and (65) Cybele have shown the presence of an absorption feature at 3.1 μm reported to be directly linked to surface water ice. We searched for water vapor escaping from these asteroids with the Herschel Space Observatory HIFI (Heterodyne Instrument for the Far Infrared) Instrument. While no H₂O line emission was detected, we obtained sensitive 3σ water production rate upper limits of Q(H₂O)< 4.1×10²⁶ molecules s⁻¹ for Themis and Q(H₂O) <7.6 × 10²⁶ molecules s⁻¹ for the case of Cybele. Using a thermophysical model, we merged data from the Subaru/Cooled Mid-Infrared Camera and Spectrometer and the Herschel SPIRE (Spectral and Photometric Imaging Receiver) instrument with the contents of a multi-observatory database and thus derived new radiometric properties for these two asteroids. For Themis, we obtained a thermal inertia G = 20 ⁺²⁵_-10 J m⁻² s^−1/2 K⁻¹, a diameter 192 ⁺¹⁰_-7 km, and a geometric V-band albedo p_V=0.07±0.01. For Cybele, we found a thermal inertia G = 25⁺²⁸_-19 J m⁻² s^−1/2 K⁻¹, a diameter 282±9 km, and an albedo pV=0.042±0.005. Using all inputs, we estimated that water ice intimately mixed with the asteroids’ dark surface material would cover <0.0017% (for Themis) and <0.0033% (for Cybele) of their surfaces, while an areal mixture with very clean ice (Bond albedo 0.8 for Themis and 0.7 for Cybele) would cover <2.2% (for Themis) and <1.5% (for Cybele) of their surfaces. Based on these very low percentage coverage values, it is clear that while surface (and subsurface) water ice may exist in small localized amounts on both asteroids, it is not the reason for the observed 3.1 μm absorption feature.